1. Field of the Invention
The present invention relates to methods and systems for testing data signal amplifiers, and in particular, to testing data signal amplifiers capable of multiple output signal power levels for minimizing power consumption.
2. Related Art
The current generation (3G) of cellular telephones offer, among other things, faster connection speeds to the internet. These higher connection speeds are achieved by more complex modulation and stricter requirements for accurate signal power levels. At the same time, since such devices are usually powered by batteries, these devices also need to minimize their power consumption.
Accordingly, to meet these conflicting requirements of high performance while consuming the lowest possible amount of power, these devices use many techniques to minimize power consumption, including minimizing current consumption in the radio frequency (RF) circuitry. Examples include dynamically changing bias current in the output power amplifier such that the bias current scales (e.g., higher or lower) in accordance with the transmitted signal power. Alternatively, or in some instances in addition, the power supply voltage to the output power amplifier can be controlled using a switching DC-DC converter to reduce the power supply voltage when transmitting at lower output signal power levels. While these techniques do help improve battery life, they also make device behavior more difficult to calibrate, since multiple power control parameters are changing as the output transmitted signal power changes. Accordingly, a more complex calibration technique is required.
Such calibration, which is done during performance testing of the device following its manufacture, has either been an iterative process or based on simple assumptions of the behavior of the device under test (DUT). The former technique will provide for more thorough testing and calibration, but at the cost of test speed, while the latter will often provide reasonable test results, but not optimal, since the test results, based on assumptions, may not be sufficiently accurate in view of the strict performance requirements.
Referring to FIG. 1, the output power amplifier 10 amplifies the outgoing data signal 11a to produce the amplified data signal 11b to be transmitted. The amplifier 10 receives its power 13b from a power source 12 controlled by a bias signal 13a. Accordingly, as discussed above, the power level of the transmitted signal 11b can be controlled, at least, by controlling the level of the input signal 11a to the power amplifier 10, as well as the DC power 13b provided to the amplifier 10 by the power source 12. As discussed above, this DC power 13b can be controlled by controlling its current or voltage in accordance with the bias control signal 13a. 
Referring to FIG. 2, as discussed above, the RF output power (i.e., transmitted signal power delivered to the antenna) is a function of the power of the input signal 11a to the power amplifier 10 and the bias setting for the DC power 13b provided to the power amplifier 10 (FIG. 1). The two curves represent two different bias settings, with the lower curve resulting from a lower bias setting and the higher curve resulting from a higher bias setting. As can be seen, at lower power levels, output power verses input power is approximately linear. However, at higher levels, power variations differ significantly between the two bias settings.
A common test technique is to perform calibration in two or more steps. One step might be varying the power of the outgoing signal 11a provided to the amplifier 10, while maintaining fixed power supply voltage and bias current. A second step could be varying the power supply voltage while maintaining fixed input signal power and bias current. A third step can then be varying the bias current while maintaining fixed input signal power and power supply voltage. As a result, amplifier performance has been characterized with respect to two or three control parameters, from which expected performance can be inferred or extrapolated, based on the observed relationships between signal power, power supply voltage and bias current. However, such expected future performance is based on one test parameter with influences based on power supply voltage or bias current assumed to be consistent from one device to another.
While this may yield acceptable results for many devices, with increasingly strict performance requirements, it is unlikely that sufficiently low failure rates during actual operations can be achieved, since the characterized performance does not accurately model actual performance.
For example, maintaining constant bias current while varying the power of the amplifier input signal 11a, the operating temperature of the amplifier 10 may be higher than that to be expected during actual operation, since the bias setting will likely be higher than that used when battery power conservation measures are being followed. While it is possible to model the expected amplifier temperature, there will nonetheless be variations among devices. Further, since the power variances are applied quickly during testing, the internal temperature of the amplifier 10 may vary during these variances, while the system temperature measured at a different location within the device will not be affected significantly due to the fast testing. Accordingly, the testing operation will not accurately simulate normal operation.